The present invention relates generally to mechanical shaft seals, and more particularly to a mechanical shaft seal which is resistant to electrokinetic corrosion.
Electrokinetic corrosion is defined and discussed in an article by T. R. Beck, et al., Am. Soc. Mech. Eng. J. Basic Eng., Vol. 92, December, 1970, at p. 782, and the definition therein is incorporated herein by reference.
Electrokinetic corrosion of a solid member can occur when there is acceleration of an adjacent liquid in which electrokinetic streaming currents (I.sub.s) or static electricity are generated. This will be discussed in greater detail below.
A shaft seal is generally employed in connection with a shaft and a housing within which the shaft rotates. Part of the shaft is immersed in liquid. There is a clearance between the outside of the shaft and the inside of the housing, and the shaft seal is used to prevent the leakage of liquid through that clearance.
The shaft seal generally comprises a stationary first annular seal element associated with the housing and through which the shaft extends. A second annular seal element is fixedly mounted on the shaft for rotation therewith. Each of the seal elements has a respective mating surface, and spring means are employed to urge the mating surfaces together to provide rubbing engagement between the mating surfaces during rotation of the shaft. The shaft seal is generally non-load-bearing.
Large shaft seals are exemplified by those used in connection with submarine shafts, which may have a diameter of about two feet, for example. The liquid (sea water) adjacent a submarine shaft seal can be under tremendous pressure, depending upon the depth to which the submarine is submerged. In conventional submarine shaft seals, one of the seal elements has been made from a relatively hard material, such as silicon carbide, and the other seal element has been made of a relatively soft or conforming material, such as graphite. When the mating surfaces of the two seal elements rub together initially during rotation of the shaft, the relatively hard silicon carbide sealing element abrades the mating surface of the relatively soft graphite second seal element until the mating surface on the graphite seal element conforms to the mating surface on the silicon carbide seal element, so that there are no irregularities on the two mating surfaces, in relation to each other, on a macro scale. (As used herein, the term macro scale refers to an irregularity having a depth or height of about 1.times.10.sup.-5 meter or larger.) Thus, the mating surfaces on such a shaft seal are quite uniform in relation to each other, on a macro scale.
When an electrolytic liquid undergoes movement in relation to an adjoining surface, an electrokinetic streaming current is generated in the moving liquid. This streaming current is proportional to certain electrical characteristics of the liquid and to the shear rate or velocity gradient of the liquid relative to the surface alongside which the liquid moves.
In a mechanical shaft seal, such as a submarine shaft seal of the type described above, there is a very small gap between the mating surfaces of the shaft seal, and this gap is less than about 1.times.10.sup.-6 meter (1 micrometer). The gap contains a liquid film of sea water, which acts as a lubricant for the mating surfaces during rotation. The film of sea water has characteristics which allow an electrokinetic streaming current to be generated when the liquid film moves relative to one of the mating surfaces. When the submarine shaft is rotated, this causes movement of the liquid film relative to the two mating surfaces, and there is a velocity gradient in the moving liquid relative to each of the two mating surfaces, resulting in the generation of an electrokinetic streaming current.
As previously noted, electrokinetic corrosion occurs only when the liquid in which electrokinetic streaming currents are generated undergoes acceleration. Because the two mating surfaces in a submarine shaft seal of the type described above are relatively smooth, one would expect the velocity of the liquid at any given plane in the gap between the two mating surfaces to be relatively constant and that the liquid would not, therefore, undergo acceleration. Accordingly, even though electrokinetic streaming currents are generated in the sea water film between the two mating surfaces of a submarine shaft seal of the type described above, because one would not expect that liquid to undergo acceleration, one would also not expect electrokinetic corrosion on either of the mating surfaces of the shaft seal.
However, although the mating surfaces of the submarine shaft seal are relatively smooth, are uniform in relation to each other and have no irregularities on a macro scale, they are not absolutely geometrically smooth, and they do have local irregularities on a micro scale, i.e., irregularities having a depth or height substantially smaller than 1.times.10.sup.-5 meter (e.g., 1.times.10.sup.-6 meter or less). These local irregularities, on a micro scale, are sufficient to cause some acceleration in the liquid undergoing movement relative to the two mating surfaces, in turn producing a current density which is normal or perpendicular to the walls or mating surfaces of the shaft seal. This normal current density (i.sub.n) has two components. One of these components is fluid conduction current (i.sub.f) and the other component, in the case of a seal element composed of a conductive material such as graphite or metal, is wall current (i.sub.w). The presence of wall current can cause electrokinetic corrosion of the wall or mating surface adjacent the liquid undergoing acceleration, and this in time can produce sufficient wear on that mating surface eventually to allow leakage of liquid through the shaft seal.
In a conventional submarine shaft seal of the type described above, there is sufficient wear under normal usage to produce a leakage in about 2-12 months, whereas several years of service without leakage is desired.